Scilab Function
Last update : February 1998

ode_root - ordinary differential equation solver with root finding

### Calling Sequence

  `y,rd[,w,iw]=ode("root",y0,t0,t [,rtol [,atol]],f [,jac],ng,g [,w,iw])`

### Parameters

- `y0`: real vector or matrix (initial conditions).
- `t0`: real scalar (initial time).
- `t`: real vector (times at which the solution is computed).
- `f`: external i.e. function or character string or list.
- `rtol,atol`: real constants or real vectors of the same size as `y`.
- `jac`: external i.e. function or character string or list.
- `w,iw`: real vectors.
- `ng`: integer.
- `g`: external i.e. function or character string or list.

### Description

With this syntax (first argument equal to `"root"`) `ode` computes the solution of the differential equation `dy/dt=f(t,y)` until the state `y(t)` crosses the surface `g(t,y)=0`.

`g` should give the equation of the surface. It is an external i.e. a function with specified syntax, or the name of a Fortran subroutine or a C function (character string) with specified calling sequence or a list.

If `g` is a function the syntax should be as follows:

    z=g(t,y)

where `t` is a real scalar (time) and `y` a real vector (state). It returns a vector of size `ng` which corresponds to the `ng` constraints. If `g` is a character string it refers to the name of a Fortran subroutine or a C function, with the following calling sequence: `g(n,t,y,ng,gout)` where `ng` is the number of constraints and `gout` is the value of `g` (output of the program). If `g` is a list the same conventions as for `f` apply (see ode help).

Ouput `rd` is a `1 x k` vector. The first entry contains the stopping time. Other entries indicate which components of `g` have changed sign. `k` larger than 2 indicates that more than one surface (`(k-1)` surfaces) have been simultaneously traversed.

Other arguments and other options are the same as for `ode`, see the ode help.

### Examples

    // Integration of the differential equation
    // dy/dt=y , y(0)=1, and finds the minimum time t such that y(t)=2
    deff("[ydot]=f(t,y)","ydot=y")
    deff("[z]=g(t,y)","z=y-2")
    y0=1;ng=1;
    [y,rd]=ode("roots",y0,0,2,f,ng,g)

    deff("[z]=g(t,y)","z=y-[2;2;33]")
    [y,rd]=ode("roots",1,0,2,f,3,g)

### See Also

`dasrt`, `ode`,